Contemporary magnetic hard disk drives typically include a rotating rigid storage disk and a head positioner for positioning a data transducer at different radial locations relative to the axis of rotation of the disk, thereby defining numerous concentric data storage tracks on each recording surface of the disk. The head positioner is typically referred to as an actuator. Although numerous actuator structures are known in the art, in-line rotary voice coil actuators are now most frequently employed due to their simplicity, high performance, and their ability to be mass-balanced about their axis of rotation, the latter being important for making the actuator less sensitive to perturbations. A closed-loop servo system within the disk drive is conventionally employed to operate the voice coil actuator and thereby position the heads with respect to the disk storage surface.
The read/write transducer, which may be of a single or dual element design, is typically deposited upon (or carried by) a ceramic slider structure having an air bearing surface for supporting the transducer at a small distance away from the surface of the moving medium. Single write/read element designs typically require two-wire connections while dual designs having separate reader and writer elements require two pairs of two-wire connections. Magnetoresistive (MR) heads having separate inductive write elements in particular generally require four wires. The combination of an air bearing slider and a read/write transducer is also known as a read/write head or a magnetic recording head.
Sliders are generally mounted to a gimbaled flexure structure attached to the distal end of a suspension's load beam structure. A spring biases the load beam and the head towards the disk, while the air pressure beneath the head developed by disk rotation relative to the slider pushes the head away from the disk. The gimbal enables the slider to present a "flying" attitude toward the disk surface and follow its topology. An equilibrium distance defines an "air bearing" and determines the "flying height" of the head. By utilizing an air bearing to support the head away from the disk surface, the head operates in a hydrodynamically lubricated regime at the head/disk interface rather than in a boundary lubricated regime. The air bearing maintains a spacing between the transducer and the medium which reduces transducer efficiency. However, the avoidance of direct contact vastly improves the reliability and useful life of the head and disk components. Demand for increased areal densities may nonetheless require that heads be operated in pseudo-contact or even boundary lubricated contact regimes, however.
Currently, nominal flying heights are on the order of 0.5 to 2 microinches. The magnetic storage density increases as the head approaches the storage surface of the disk. Thus, a very low flying height is traded against device reliability over a reasonable service life of the disk drive. At the same time, data transfer rates to and from the storage surface are increasing; and, data rates approaching 200 megabits per second are within practical contemplation.
The disk drive industry has been progressively decreasing the size and mass of the slider structures in order to reduce the moving mass of the actuator assembly and to permit closer operation of the transducer to the disk surface, the former giving rise to improved seek performance and the latter giving rise to improved transducer efficiency that can then be traded for higher areal density. The size (and therefore mass) of a slider is usually characterized with reference to a so-called standard 100% slider ("minislider"). The terms 70%, 50%, and 30% slider ("microslider", "nanoslider", and "picoslider", respectively) therefore refer to more recent low mass sliders that have linear dimensions that are scaled by the applicable percentage relative to the linear dimensions of a standard minislider. Sliders smaller than the 30% picoslider, such as a 20% "femtoslider", are presently being considered and are in early development by head vendors. As slider structures become smaller, they generally require more compliant gimbals; hence, the intrinsic stiffness of the conductor wires attached to the slider can give rise to a significant undesired mechanical bias effect.
Trace interconnect arrays are now being proposed to support or aid in supporting the slider next to the data storage surface, and to connect read and write elements of the head with external circuitry. Two conductor paths are typically required for the write element, and two separate conductor paths are required for the read element, of the magnetic head. The interconnect array, typically formed on a polyimide film substrate, may extend from the slider to a preamplifier/write driver circuit, either directly, or via one or more intermediate interconnect trace arrays. These designs typically include trace segments extending from the flexure to a signal connection point which may be located on the side of the rotary actuator, for example. Since these trace conductor interconnect arrays are flat, and are precisely formed printed circuits upon plastic film substrates, they tend to have more predictable mechanical properties than discrete wire conductors used in the past, thereby reducing tolerances in manufacturing and operation.
In transmission lines and interconnects of the type under contemplation, it is important to reduce the effect of the interconnect on the source (preamp for write element transducer for read element, of magnetic recording head, for example). The inductance and capacitance parameters of the trace array introduce a phase-change in the current/voltage waveforms, and most designs are made to minimize undesired effects of inductance and/or capacitance upon overall circuit performance.
One method to achieve reduced effect of inductance and/or capacitance is to ensure that reactive components of the interconnect are minimal. Usually, there is a trade-off between the inductance and the capacitance, since reducing inductance means moving the conductors closer together which increases the inter-conductor capacitance. Once conductor spacing is fixed at a minimum distance limited by manufacturing tolerances, the inductance can be further reduced by increasing the conductor width, which also results in a slight increase in inter-conductor capacitance, and a potentially significant increase in conductor-to-ground capacitance if a ground plane is nearby.
Since the amount of space available for the trace interconnect array is limited, the conductors can only be widened to a certain extent. This tradeoff between reduced inductance and increased capacitance results in a very inefficient trace interconnect array design. Because of skin-effects and/or proximity-effects present at high signal frequencies, the signal current is pushed toward the crosssectional perimeter of the conductor, and the conductor cross-section is thus not utilized for carrying the signal current to the fullest extent. Therefore, the current distributes itself around the perimeter of the conductor cross-section and the resultant resistance and inductance are perimeter effects, rather than crosssectional area effects. Thus, reduction in inductance by widening the conductor follows a law of diminishing returns.
There are established methods which attempt to address or solve this problem. It has already been shown that the current distribution can be greatly improved by splitting wide conductors into a number of parallel conductive segments to reduce the resistance and inductance of the interconnect, see commonly assigned, U.S. patent application Ser. No. 08/726,450 filed on Oct. 3, 1996, now U.S. Pat. No. 5,717,547, for "Multi-Trace Transmission Lines for R/W Head Interconnect in Hard Disk Drive" by James A. Young, the disclosure thereof being incorporated herein by reference. While this approach has the advantage of reducing inductance, it results in increased interconnect capacitance, and increased implementation complexity, including multiple layers and bridge vias, or addition of bridging jumpers, at both ends of the conductor trace array.
Also, a microstrip configuration which uses perfectly registered conductors results in very high values of capacitance for reductions of inductance. Reducing the capacitance requires very thin conductors which increases the resistance by a few orders of magnitude. Additionally, while multi-layered geometries can realize certain improvements in electrical characteristics, multi-layers are more expensive to fabricate than single-layered geometries, see e.g. the present inventor's commonly assigned U.S. patent application Ser. No. 08/720,833 filed on Oct. 3, 1996, for "Suspension with Multi-Layered Integrated Conductor Trace Array for Optimized Electrical Parameters", the disclosure thereof being incorporated herein by reference.
Thus, a hitherto unsolved need has remained for a trace interconnect array having more effectively controlled inductance and capacitance characteristics.